This invention relates to cutting tools. More particularly the invention relates to tools for making a helical incision around the inner surface of hollow bodies, such as tubular hollow bodies.
Such tools have been proposed in which the cutting blade is mounted on a shaft arranged to project within the hollow body parallel to the axis thereof the cutting blade projecting outwardly from the shaft so as to impinge upon the internal surface of the hollow member. Appropriate axial and radial relative movement between the shaft and the member to be cut then results in the formation of a helical cut. Usually the relevant movement is caused by rotating the hollow member with the cutting blade engaging the internal surface of the body, and at the same time axially moving the shaft carrying the cutting blade.
Such a cutting operation is required, for example, in the production of electroweld couplings for thermoplastic pipes. In this case a sleeve of thermoplastic material is provided, in one type of such coupling, with a heating wire inserted in helical fashion within the internal surface thereof. The insertion of the wire can be achieved by incising a helical cut or groove around the internal surface of the sleeve, and then inserting the heating wire into the cut or groove so formed. Couplings of this kind can utilise a sleeve of a very wide range of sizes, from say 20 mm internal diameter up to 200 mm internal diameter.
It has been found in practical usage of electroweld couplings of the kind mentioned hereinabove that for couplings intended to connect together two pipes, whilst there is need for a relatively close pitched helical formation of the heating wire adjacent each end of the coupling so as to achieve satisfactory melting and fusion of the coupling sleeve and pipes during the fusion process, there is no need for the same close pitched spiral across the central portion of the coupling sleeve, and indeed the provision of a close pitched spiral throughout the total length of the sleeve can be disadvantageous in that greater electrical power is required for fusion than is necessary on an optimum efficiency basis, and that an unnecessary length of heating wire is incorporated in the sleeve.
It has been proposed that the cutting tool should be programmed so as to form a close pitched helical incision adjacent each end of the coupling sleeve, whilst a much wider pitch should be provided over the central portion of the sleeve such that, for example, the tool traverses the central portion in 1 or 1.5 revolutions of the coupling sleeve relative to the tool.
A difficulty arising from such a proposal however is that the cutting tool must be capable of cutting at a variable helix angle. This in practice would require a "backing off" or tapering of the cutting blade from its leading to its trailing edge to enable the different helix angles to be cut without the trailing edge of the tool fouling on a side of the incision made. With the dimensions of the range of pipes concerned, and the consequent dimensions of the tool and its cutting blade, adequate backing off to achieve the variation in helix angle is difficult to achieve and results in a substantially weakened cutting blade.
It is an object of the present invention to overcome or at least substantially reduce the above mentioned problem.